CEEN 341 - Lecture 19 - Intro to Shear Strength and the Direct Shear Test
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- Опубліковано 14 чер 2024
- This lecture introduces the basic concepts of shear strength of soil. Soil friction is described, and the friction angle is derived. The Mohr-Coulomb failure criteria are introduced, which state that soil shear strength is a combination of cohesion and friction. The direct shear test is introduced and explained. An an example calculation with the direct shear test is performed to develop a Mohr-Coulomb failure envelope.
Learning Objectives:
1) Derive and/or explain what the the friction angle is in the soil.
2) Derive the Mohr-Coulomb equation for shear stress in the soil, and explain when failure occurs in the soil.
3) Explain the operation of the direct shear test and list some of its advantages and/or disadvantages.
4) Using direct shear test results, compute the friction angle of a sand.
Critical Questions to Consider:
1) What are the two sources to soil shear strength?
2) Why can't a Mohr circle ever cross the Mohr-Coulomb failure envelope? What happens once a point on the circle touches the envelope? How does the point of tangency where the Mohr circle touches the failure envelope relate to the orientation of the failure plane in the soil?
3) What are some advantages and disadvantages of the direct shear test?
4) What does it mean for a shear test to be a "drained" shear test? How do you think this would differ from an "undrained" shear test, which we will talk about in the next lecture or two?
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Great presentation. Thank you professor Franke!
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Thank you so much sir!! I couldn't grasp the concepts and had literally lost complete interest in soil mechanics course of my curriculum. But your videos have been extremely helpful . Thanks a lot sir!!!
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About the DSS test, why it cant be used for fiding undrained strengh?
I would like to know a little bit more about the limitations of DSS test.
The direct shear device (DS) does not have a membrane surrounding the soil, and therefore cannot be used to perform undrained tests. On the other hand, the direct simple shear device (DSS) DOES have a membrane and is capable of performing undrained tests. Perhaps you mistook my refence to the DS test as the DSS test.
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Does anyone understand how the angle Professor Franke found on 20:11? I don't understand why.
an external angle at any corner with an extension of a line will be a summation of two other angles of the triangle.
two radius of the circle therefore the triangle has two equal corners (0cr) . then the external angle 180-(180-20cr)
Hi prof, im really into geotechnics but it's quite hard to really understand the concentps.Could you be more specific about the symmetrie of mohr-coulomb criteria,the symmetrie means that the element fails at to planes? one above and one below? or at one only and the direction is govern by the nature of the element?
By definition, a circle is symmetric. Mohr's circles are symmetric about their center. For simple loading assumptions, the stresses that define the circle are symmetric on both sides of the sigma axis (i.e., the x-axis). In advanced soil mechanics, you will learn that the failure envelope is also "sort-of" symmetric about the sigma axis, meaning that there is a failure envelope on the negative side of the sigma axis that represents the failure state for tension. It has a different friction angle than the failure envelope for compression though, so it's not really symmetric.
at 43:28, while calculating tau, a contact area between soil decrease. shouldn't we consider?
You could, but the change in area is small enough and its effect is negligible such that most engineers ignore it.
@@officehours4028 but even if sample get failed at a displacement of 0.5 cm..then reduction in area is 3 sq.cm. if we are using 6 cm * 6 cm box..i think which is considerable..but yeah..considering full area will give us conservative results..lower tau and lower phi..👍..thank you.😊
3:00 That house is horrified.
HAHAHA....yes, it is